FIGURE 2.

Calcium pectate cycle. Only two calcium pectate cross-links are shown (ovals in anti-parallel pectate molecules, left side of figure in black), but the same principles apply to larger numbers of calcium pectate cross-bridges in the wall. Turgor pressure is high enough to distort the egg box in one of the pair, weakening its bonds with calcium (left pectate in pair). New pectate from the cytoplasm (dashed red arrow) is undistorted and preferentially removes calcium from the distorted pectate (step 1, red). The load-bearing pectate relaxes after its cross-bridging calcium is removed. The wall elongates incrementally, shifting the load to the other member of the pectate pair, which distorts. The remaining steps 2–4 follow (see text) and result in a cycle. Additional calcium pectate is deposited in the wall as part of the cycle (step 3, green). The net result is elongation plus wall deposition. Note that in Chara the cycle occurs in the medium in which the cells are grown (0.6 mM Ca²⁺). Also note that the rate of growth depends on the rate of pectate release from cytoplasm to wall by exocytosis (red and green dashed arrows).